1. Field of the Invention
The present invention relates to improving natural lighting within buildings and more particularly to daylight harvesting for building interior illumination. More particularly, this invention relates to daylighting elements of a building such as glazed wall openings, windows, roof windows and skylights, as well as to various devices and glazing structures used for admitting and distributing daylight into the interior of a building, such as light shelves, light redirecting blinds or louvers, prismatic films and panels, and transparent plates and light guides employing total internal reflection surfaces.
2. Description of Background Art
Various optical structures for redistributing daylight into building interiors are known. At least some of such prior art devices employ planar transparent plates of glass or plastic materials which include reflective surfaces embedded between the opposing sheet surfaces and configured to reflect light by means of a total internal reflection (TIR). The use of TIR structures generally allows for much larger bend angles compared to refractive structures such as prismatic sheets or films. Large bend angles are particularly important for redistributing daylight in the interior of a building so that at least a portion of the incident daylight could be directed towards the upper portions of the interior, such as the ceiling of a room.
For example, one such light redirecting structure employing internal TIR surfaces is disclosed in U.S. Pat. No. 737,979 which shows a glass plate including a series of slots made in its body. The angle of these slots is such that daylight coming from any given principal direction from outside is reflected from the surface of the slot and is hereby redirected from its original propagation path. Another light redirecting structure is disclosed in U.S. Pat. No. 6,424,406 which describes optical diffuser plates made from transparent plastics and employing either thin strips of another plastic or hollows in the respective plates to deflect light.
U.S. Pat. No. 7,416,315 discloses a faceted reflector which includes a plurality of prismatic reflectors embedded in a carrier and reflecting light by total reflection at a part of the cavity interfaces. In U.S. Pat. No. 6,616,285, total reflection surfaces are formed by merging two optical bodies each having surface groves which interpenetrate into one another when such bodies are placed face-to face. U.S. Pat. No. 5,880,886 shows V-section grooves formed in a major face of a substantially flat and planar optical element. U.S. Pat. No. 4,557,565 discloses a planar solid transparent light deflecting panel or plate for transmitting sunlight into the interior of a building. The panel or plate is formed of a plurality of parallel identically spaced apart triangular ribs on one face. The ribs have specially selected slopes to totally internally reflect light when such panel or plate is placed over an opening such as window.
On the other hand, various methods of making the light redirecting TIR structures in such transparent plates have been proposed. For example, U.S. Pat. No. 4,989,952 discloses a method for producing a transparent light deflecting panel comprising making a series of parallel cuts in a sheet of transparent solid material with a laser cutting tool. Such panel can be positioned in an opening in the facade of a building to deflect incident daylight towards the ceiling thereby improving the natural lighting within the building. The transparent sheet is commonly acrylic and the laser tool is a carbon dioxide (CO2) laser.
CO2 laser cutting of rigid PMMA panels has generally demonstrated its ability to form relatively narrow channels suitable for TIR and light redirection purposes. However, it is fairly difficult to make the width of the channels less than 150 micrometers (μm) or so, given the finite diameter of the beam of CO2 lasers, fundamental and practical limitations of beam-focusing optics and hard-to-control excessive material evaporation in the process of ablative material removal. Since the minimum attainable width of the laser cut channels relates to the minimum acceptable thickness of the acrylic panel, due to the optical efficiency and structural considerations, such panels are more practical in thicknesses of 6 mm or more. However, a 6-mm acrylic panel can be fairly heavy for the typical sizes of wall windows which can translate into substantial material costs and difficulties in fixing the panel in a suspended position near the window.
U.S. Pat. No. 6,580,559 describes a method of forming internal TIR structures in transparent panels made from glass-like thermoplastic material such as PMMA by inducing parallel crazes in the thermoplastic material. An organic solvent is applied on the panel surface while a tensile stress is applied to a panel which results in generation of wedge-shaped deformations (crazes) which propagate within the material. However, such method of forming internal TIR structures offers little control over the spacing, depth and extent of the crazes, as well as can substantially compromise the structural integrity or rigidity of the panel.
Mechanical surface slitting of a sheet-form material using razor-sharp blades can be one way of making TIR channels or facets within the bulk material of an optically clear sheet. However, slitting of hard and brittle optical plastics such as polycarbonate, polystyrene, rigid PVC or polycarbonate with a blade is generally unfeasible considering the lack of sufficient plasticity of such materials and is usually ruled out for daylighting devices employing TIR surfaces. The blade penetration into such materials induces material chipping and/or smearing which makes it nearly impossible to form a smooth and narrow cut suitable for TIR functionality even assuming the ideal sharpness of the blade. Additionally, the blade thickness brings its own limitations on the minimum width of the cut in a rigid material.
The use of rigid transparent plates or panels as a substrate in which TIR surfaces can be formed is also deficient in that such devices are inherently inflexible which makes them difficult or impractical to be utilized in lightweight and compact daylighting systems. It is also difficult or impractical to incorporate rigid sheets or plates in retractable window coverings or use them for lamination onto a window pane or other substrates used in glazing.
Many daylighting systems would benefit from employing a sheet-form light redirecting TIR structure that can be sufficiently thin and flexible, resembling the behavior of a fabric, and that can be bent to tight radii or laminated onto a surface. Furthermore, many daylighting applications would also benefit from an efficient and low-cost method of making such flexible TIR sheet-form structures.
Accordingly, prior efforts have failed to provide a practical and cost-effective solution for admitting daylight into building interiors and providing uniform and efficient natural illumination using a relatively thin sheet material. These needs and others are met within the present invention, which provides an improved sheet-form structure for illuminating building interiors with sunlight and also provides a method of making the same. The improved sheet-form structure employs internal TIR surfaces to efficiently redirect light and is also thin, flexible and has a fabric-like behavior, all of which finds utility in various daylighting devices and systems.